1. Field of the Invention
The field of the invention is that of vibrating micro-systems, and notably, but not exclusively, micro-sensors whose measurement principle is based on the oscillation frequency of an oscillating mechanical system, using a beam or beams, a membrane or membranes, formed by means of micro-machined electro-mechanical structures, known as MEMS, placed in an cavity under vacuum.
The invention is more particularly applicable to precision sensors, such as those carried onboard aircraft and used for guidance or piloting assistance, supplying indications of pressure, acceleration or angular velocity. These sensors must supply a measurement whose actual level of accuracy must be known at each moment in time, because these measurements are used for critical or essential functions for the safety or the mission of the aircraft. For example, a false indication of static pressure leads to an erroneous indication of altitude. It will be well understood to what extent it is essential that the flight management system and the pilot are aware of this error.
2. Description of Related Art
Vibrating resonator micro-sensors are microstructures containing materials such as quartz and silicon, which use the resonance of a (or sometimes several) vibrating elements. The resonator is typically a vibrating strip or a beam, enclosed within a chamber under a controlled atmosphere, typically under vacuum.
The formation of these microstructures is very advantageous because they are obtained by collective fabrication processes using the usual fabrication steps for electronic integrated circuits, and allow the fabrication of very small and inexpensive components.
The well-known principle of the measurement is as follows: a particular resonance mode of the vibrating resonator is used, controlled by an excitation circuit comprising an automatic gain control loop. An external physical quantity applied to the vibrating resonator is thus converted into a variation of the resonance frequency or a variation of the amplitude of the vibrational motion. This variation in resonance frequency or in amplitude allows the applied stress to be measured.
Pressure micro-sensors, micro-accelerometers, or micro-gyrometers are fabricated according to this principle such as notably respectively described in the patent applications FR0215599, FR9202189 and FR0507144.
FIG. 1 illustrates very schematically the functional elements of a micro-sensor with closed-loop control. A resonator 10 with one or more vibrating element(s) 11 is included in a closed-loop electronic circuit 20 for automatic gain control. The loop 20 typically comprises a signal detection circuit 21, and an automatic gain control circuit 22 AGC. The detection circuit 21 typically comprises a signal amplifier and a bandpass filter, and is characterized by a gain Gd. This detection circuit 21 detects an electrical signal, representative of the oscillation of the resonator, for example a current, and supplies at the output a corresponding electrical signal y(t) after filtering and amplification; this signal represents the oscillating motion of the resonator.
This electrical signal y(t) and an external amplitude setpoint C can be applied to the input of the automatic gain control circuit 22, which supplies at the output a corresponding electrical excitation signal E applied to the resonator. The circuit 22 and the setpoint C are designed to make the vibrating element (or the vibrating elements) of the resonator vibrate with predetermined oscillation amplitude A0.
In the absence of any stress, for example under a zero external pressure for a pressure sensor, or in the case of a zero acceleration for an accelerometer, the vibrating element oscillates at its natural resonance frequency Fp0. In the presence of a stress of value σ, the resonance frequency will vary and will take a value Fp, and it is the difference in frequency Fp−Fp0 on which the measurement is based. The measurement is, in practice, supplied by a signal processing device 30, generally a digital processing system which samples the signal y(t) and which analyses it by any known techniques in order to determine the frequency Fp and to supply the corresponding measurement M of the physical stress sought (pressure, acceleration, angular velocity), based on the variation between the measured frequency Fp and the resonance frequency Fp0 without stress. This variation in frequency is thus an image of the applied stress.
For the fields of application indicated hereinabove, the sensors must have an excellent performance in terms of sensitivity, accuracy and scale factor. The guarantee of the precision of the measurement of a micro-sensor is intimately linked to the maintenance of the vacuum within the sensor. Indeed, the resonator of the sensor must have a very high quality factor of the resonance, of the order of a few tens of thousands (104) to several millions (106), but a deterioration in the vacuum corresponds to a deterioration of the quality factor, and this deterioration of the quality factor results first of all in a deterioration of the signal-to-noise ratio and finally in a loss of precision of the measurement.
Under operational conditions, the structure of the micro-sensors may get degraded. Notably, a partial loss of vacuum in the sensor may occur, leading to a loss of precision in the measurement that the user has no means of detecting. This is a major drawback of these micro-sensors. In fields such as that of avionics, it is indeed essential to know, at all times, whether the measurement supplied is reliable or not.
For example, for an onboard pressure sensor in an aircraft, it could be envisioned to verify the measurement of the sensor by other sources such as the inertial data, the GPS positioning data, or the radio-altimetric height data. However, these sources have a limited availability or an insufficient precision, and they moreover only indirectly represent the measurement that it is desired to verify.
According to the rules of the art, a verification by redundancy can be set up with a second sensor, similar to the first, which is very unlikely to fail at the same time as the first; but in case of discrepancy, then a third sensor is needed in order to determine which is the defective sensor.
Lastly, these solutions do not provide a diagnostic for a failure nor an advanced warning of a degradation.
Generally speaking, it is desirable to be able to monitor, during operation, the performance characteristics of a micro-sensor and to detect the fact that the measurement has gone outside of an acceptable tolerance.